The present invention relates to processes useful in the manufacture of polyamides and more particularly relates to a process for increasing the relative viscosity of polyamides while reducing thermal degradation.
For the manufacture of high strength nylon fibers and other uses, high quality, linear polyamides with high relative viscosity (RV) are often desired. Known processes for increasing the RV of a polyamide typically employ a phosphorous acid catalyst in the polyamide together with elevated temperatures and less than the equilibrium level of H.sub.2 O in contact with the polyamide to promote the amidation reaction between terminal carboxylic and amino groups of the polymer. At relatively low RV levels, the length of the polymer chains is increased in a predominantly linear fashion to achieve an increase in RV.
With some polyamides, particularly polyadipamides such as poly(hexamethylene adipamide), problems arise due to thermal degradation which results in branched polymer and increased "gel" formation when known processes are used for increasing the RV to higher levels. "Gel", a very high molecular weight, cross-linked polymer, forms in the polymer and can deposit on surfaces in contact with the molten polymer during polymerization or in subsequent handling such as in transfer lines and spinneret packs in melt spinning operations. Since the gel is essentially insoluble and often can only be removed from such equipment using extreme measures such as by burning, gel formation increases the cost of using such high RV polymer. Moreover, polymer containing particles of gel is of generally lower quality and is less suited for the production of high strength fiber.
When it is attempted to suppress thermal degradation and decrease gel formation by adding a base to the polymer, the catalytic effect of some otherwise very effective phosphorous acid catalysts, such as phenyl phosphinic acid, is decreased to a sufficient extent that high RV polymer cannot be made efficiently. In a continuous polymerization process, this results in a limitation of throughput, increased sensitivity to process changes and shorter equipment life due to the more extreme conditions which are required. In addition, due to the slower reaction, the polymer may not have reached equilibrium RV before a subsequent operation such as melt-spinning which can result in a variable product.